This invention relates to a socket structure for mounting an ultrasonic gas flow measuring device with respect to a gas flow pipe, and more particularly to a socket structure which is provided at a gas flow pipe for mounting, with respect to the pipe, a device for propagating ultrasonic waves through gas flowing via the pipe in order to measure the flow velocity and flow rate of the gas.
It is well-known in the art that when ultrasonic waves are propagated through gas flowing via a pipe, the propagation velocity of the ultrasonic waves vary depending upon the flow velocity of the gas. Such a principle has been utilized to develop an ultrasonic gas flow measuring device which is adapted to propagate ultrasonic waves through gas flowing via a pipe in order to measure the flow velocity of the gas, thereby reducedly calculating the flow rate of the gas.
Such an ultrasonic gas flow measuring device is typically constructed in such a manner as shown in FIG. 1. More particularly, a gas flow pipe 10 through which gas is flowed is provided with a pair of cylindrical sockets 12, which are arranged on the pipe 10 in such a manner as to be spaced from each other at an angle of 180 degrees in the circumferential direction of the pipe 10 and oblique at an angle of .theta. with respect to an axis of the pipe 10 so as to be linearly spaced at a predetermined distance 1, thereby being obliquely opposite to each other and fixed on the pipe 10 by a suitable means, such as welding or the like.
In each of the sockets 12 constructed as described above, a probe head 14, which is an ultrasonic transmit-receiving element acting as an ultrasonic transreceiver and fixedly mounted at one end thereof on a distal end of a terminal box 16, is securely received. Each of the probe heads 14 is inserted in the socket 12 in a manner to be aligned with an axis of the socket 12 and airtightly received therein. Also, each of the probe heads 14 is so positioned that one end thereof is aligned centrally with an inner surface of the pipe 10. Further, the probe heads 14, as described above, are arranged so as to be spaced at a predetermined distance 1 from each other. The ultrasonic gas flow measuring device also includes a signal supply cable 18 covered with a protection tube 20 and having one end connected through the terminal box 16 to the probe head 14 and the other end connected to a common controller 22.
The ultrasonic gas flow measuring device constructed as described above is operated in such a manner that the probe heads 14 alternately and repeatedly carry out the transmitting and receiving of an ultrasonic pulse signal controlled by the controller 22 through the distance 1 therebetween, so that the propagation time of the ultrasonic pulse signal propagated through gas flowing via the pipe 10 may be measured to determine the flow velocity and flow rate of the gas.
More particularly, during the alternate transmitting and receiving of ultrasonic waves between the probe heads 14, the propagation time of the ultrasonic waves through gas flowing via the pipe 10 when the direction of flow of the gas coincides with that of propagation of the ultrasonic waves is measured as t.sub.1 and the propagation time of the ultrasonic waves when the direction of the gas flow is opposite to that of propagation of the ultrasonic wave is measured as t.sub.2, and the so-measured propagation times t.sub.1 and t.sub.2 are respectively applied to the following equations (1) and (2): ##EQU1## The equations (1) and (2) are used for obtaining the following equation (3): ##EQU2##
wherein V=flow velocity (m/s),
L=distance between probe heads (m) PA1 .theta.=angle of propagation axis of ultrasonic wave with respect to axis of pipe, and PA1 C=propagation velocity.
Such an ultrasonic gas flow measuring device does not cause pressure loss of gas flowing via the pipe because it is free of any obstacle to the flowing of the gas. Also, it has another advantage in that it is substantially free of any movable mechanical parts. Further, it is capable of measuring the flow velocity and flow rate of gas over a wider angle with satisfactory reproducibility. Thus, this device has been extensively used in a variety of fields.
In the sockets which receive the probe heads of the ultrasonic gas flow measuring devices, an excessive gap between the socket and the probe causes gas flowing through the pipe to be disturbed, resulting in a measurement error. In order to avoid such a problem, such a gap g (FIG. 1) is generally determined to be as small as about 1.5 mm.
Nevertheless, the conventional socket has the following disadvantage. Gas flowing through the pipe 10, which generally contains much moisture or is in the form of a substantially saturated steam, is gradually decreased in temperature during the transfer through the pipe, so that the moisture may be gradually condensed on an inner surface of the pipe 10. The so-condensed moisture often adheres to or is collected at an entrance portion of the gap g between the socket and the probe head, as indicated by reference numeral 24 in FIG. 2. Alternatively, the condensed moisture enters into the probe head 14, for example, through the gap g. When the condensed moisture is interposed via the gap between the socket and the probe head to form a bridge therebetween, ultrasonic waves transmitted from one of the probe heads are propagated as shown in FIG. 3. More particularly, a part of the ultrasonic wave transmitted from one of the probe heads is propagated via a regular route to the opposite probe head as indicated by a solid line arrow X in FIG. 3, whereas the remainder of the ultrasonic wave is propagated through an irregular route, which includes the condensed water, and is indicated by a dotted arrow Y in FIG. 3. Generally, ultrasonic waves propagated through the irregular route have a propagation velocity as rapid as about 5000 m/sec, whereas those propagated through the regular route have a propagation velocity as slow as 340 m/sec. Thus, the former ultrasonic waves start to reach the other or opposite probe head in the form of a noise signal before the latter ones. This so-received noise signal makes accurate measurement of the flow velocity and flow rate of the gas impossible to carry out.
Accordingly, it would be highly desirable to provide an improved socket structure for mounting an ultrasonic gas flow measuring device with respect to a gas flow pipe.